Lately, when I open the Minecraft launcher my username (“kpreid”) is sometimes replaced with “O” (just the one letter). The password is remembered correctly — if I retype the username I can log in. If I retype the username, log in, quit, and relaunch, then it remembers correctly for a while. I have not found how to reproduce the problem.

2012-01-24: I launched MC through Steam and noticed the “O”. I immediately quit and relaunched it directly and saw “kpreid”, trying both several times to confirm the correlation. I then tried changing the Steam shortcut launch options to add a command-line argument, and that argument showed up as the login name (persistently), which suggests that perhaps Steam is generating a bogus “O” argument some of the time, or the way in which Steam launches MC results in it accessing files differently. The lastlogin file had identical contents before and after I logged in correctly.

2010-01-25: I saw a few occurrences of garbage including Steam-ish pathnames appearing in the login field.

1 Answer
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Based on my observations (see log in question), I am reasonably confident that this is a Steam bug where it adds garbage to the arguments of the shortcut-launched program.

On Jan 26, I added my username explicitly to the arguments in the shortcut properties; this has prevented the problem from occurring so far as I've noticed, so it apparently only happens if no explicit arguments are provided.